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 Post subject: Mix different drive sizes on 3par 7200
PostPosted: Wed Jun 01, 2016 9:58 am 

Joined: Thu Nov 12, 2015 10:12 am
Posts: 4
Hello All,

We have a 3par 7200 2 node with SFF M6710 disk enclosures.
I believe that disks need to be populated in M6710 disk enclosure in pairs and starting from the lowest numbered bay to the highest (please correct me if i am wrong)

Also so far we have used 900 GB 10 K SAS disks however our customer now has ordered 1.8 TB SAS 10 K disks. Please let me know if it is recommended to mix different drive sizes in this configuration even if speed is the same.
Also can different size of disks (speed, type is same) be used in the same disk enclosure or different disk enclosure for different type of disks?
Your inputs will help me plan this a lot better. I checked different manuals in HP 3PAR website, but the information is not available easily.
You help will be very much appreciated.
Thanks
J


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 Post subject: Re: Mix different drive sizes on 3par 7200
PostPosted: Wed Jun 01, 2016 11:16 am 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 454
Well.....

One very important aspect you're not mentioning is how is your 900GB disks utilized today performance wise (iops), how many 900GB disk do you have, how are they placed, how many 1.8TB disks are you adding and how are you planning to use them (dedicated environment or as a general expansion to the existing system)?

As a general start this is not the optimal way to go and depending on the missing information above this could be very bad and it could also not be a problem at all.

The only thing that could be answered definitively is that you are correct on the first part. Disks are added left to right and in pair of equal disks. (Lowest to highest would be incorrect for the M6720 LFF cage.)


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 Post subject: Re: Mix different drive sizes on 3par 7200
PostPosted: Wed Jun 01, 2016 12:28 pm 

Joined: Mon Aug 20, 2012 1:54 pm
Posts: 33
Location: Atlanta, GA
It's never "recommended" to mix disk size's, but in this case (size is the only difference) you could do it even if you are adding them to a general pool. The issue you will need to forever monitor though is hot spots should your 900 GB disks fill up before your 1.8 TB disks do. Ultimately that will happen, if you plan to use the majority of that space.
If however, your space usage is low, but your extra spindles are for performance reasons, then it might never become an issue.


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 Post subject: Re: Mix different drive sizes on 3par 7200
PostPosted: Tue Jan 31, 2017 6:02 pm 

Joined: Tue Jan 31, 2017 5:52 pm
Posts: 3
zQUEz wrote:
It's never "recommended" to mix disk size's, but in this case (size is the only difference) you could do it even if you are adding them to a general pool. The issue you will need to forever monitor though is hot spots should your 900 GB disks fill up before your 1.8 TB disks do. Ultimately that will happen, if you plan to use the majority of that space.
If however, your space usage is low, but your extra spindles are for performance reasons, then it might never become an issue.



As I understand it, the 3PAR architecture is engineered as such to avoid these very problems that are more common in traditional arrays. The wide-striping/virtualization via the chunklet technology should balance the drive utilization across the system and reduce the proliferation of hot spots.


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 Post subject: Re: Mix different drive sizes on 3par 7200
PostPosted: Wed Feb 01, 2017 5:16 am 

Joined: Mon Aug 20, 2012 1:54 pm
Posts: 33
Location: Atlanta, GA
bclements3 wrote:
As I understand it, the 3PAR architecture is engineered as such to avoid these very problems that are more common in traditional arrays. The wide-striping/virtualization via the chunklet technology should balance the drive utilization across the system and reduce the proliferation of hot spots.

Consider the scenario; you have 20 total NL disks. 10 of them are 2TB disks and 10 of them are 4TB. In a balanced system, as data is written it is spread equally across all the disks. If the 2TB disks never get full, then there should be no penalty.

However, if you continue to write data such that the 2TB disks gets full, you will still have half the capacity remaining on the 4TB disks. So the system will continue to write to the 4TB disks. Now you have half the number of spindles handling your workload. When you read data, you now have an unbalanced workload because some reads benefit from all the spindles, and some from only half the spindles. The 4TB disks will have to work harder as they are returning more reads than the 2TB disks.


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 Post subject: Re: Mix different drive sizes on 3par 7200
PostPosted: Wed Feb 01, 2017 11:50 am 

Joined: Tue Jan 31, 2017 5:52 pm
Posts: 3
zQUEz wrote:
bclements3 wrote:
As I understand it, the 3PAR architecture is engineered as such to avoid these very problems that are more common in traditional arrays. The wide-striping/virtualization via the chunklet technology should balance the drive utilization across the system and reduce the proliferation of hot spots.

Consider the scenario; you have 20 total NL disks. 10 of them are 2TB disks and 10 of them are 4TB. In a balanced system, as data is written it is spread equally across all the disks. If the 2TB disks never get full, then there should be no penalty.

However, if you continue to write data such that the 2TB disks gets full, you will still have half the capacity remaining on the 4TB disks. So the system will continue to write to the 4TB disks. Now you have half the number of spindles handling your workload. When you read data, you now have an unbalanced workload because some reads benefit from all the spindles, and some from only half the spindles. The 4TB disks will have to work harder as they are returning more reads than the 2TB disks.


I understand what you're saying, but wouldn't the 3PAR distribute the 1 GB chunklets such that the 2TB disks would not get full before the 4TB disks? For example, using 2 chunklets in the 4 TB disks for every 1 chunklet used in the 2 TB disks. So even though there are different size disks, the drive utilization would be balanced.


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 Post subject: Re: Mix different drive sizes on 3par 7200
PostPosted: Wed Feb 01, 2017 12:55 pm 

Joined: Mon Aug 20, 2012 1:54 pm
Posts: 33
Location: Atlanta, GA
What you are saying for space consumption is true, but not for performance. You could put 2 blocks of data on a 4TB disk for every 1 block onto a 2TB disk and get even space utilization. But, you still have the 4TB disks having to work twice as hard as your 2TB disks because they are 2x more likely to have to read from their platter than the 2TB disks. The issue with that is the 4TB disks don't read/write/transfer data faster than the 2TB, it just stores more. Therefore, your overall performance could be impacted as you are hitting the larger disks with more I/O.


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 Post subject: Re: Mix different drive sizes on 3par 7200
PostPosted: Wed Feb 01, 2017 12:57 pm 

Joined: Tue May 05, 2015 10:56 am
Posts: 31
That's how I understand it also. It will write twice the amount of data to the 4TB than the 2TB, causing the 4TB to be loaded more until it is rebalanced based on hot cold data.


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 Post subject: Re: Mix different drive sizes on 3par 7200
PostPosted: Wed Feb 01, 2017 1:45 pm 

Joined: Tue Jan 31, 2017 5:52 pm
Posts: 3
zQUEz wrote:
What you are saying for space consumption is true, but not for performance. You could put 2 blocks of data on a 4TB disk for every 1 block onto a 2TB disk and get even space utilization. But, you still have the 4TB disks having to work twice as hard as your 2TB disks because they are 2x more likely to have to read from their platter than the 2TB disks. The issue with that is the 4TB disks don't read/write/transfer data faster than the 2TB, it just stores more. Therefore, your overall performance could be impacted as you are hitting the larger disks with more I/O.


Okay, thanks for entertaining my questions, that's starting to make more sense to me.


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 Post subject: Re: Mix different drive sizes on 3par 7200
PostPosted: Thu Feb 02, 2017 6:27 pm 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 454
david wrote:
That's how I understand it also. It will write twice the amount of data to the 4TB than the 2TB, causing the 4TB to be loaded more until it is rebalanced based on hot cold data.


Wrong!

On the HP EVA systems and possibly others, they would fill drives equally percent-wise. The 3PAR will fill (given that there isn't a hardware imbalance) GB for GB(or more exact chunklet for chunklet).

If you fill the drives percent-wise, you will end up with a scenario that your 2x sized drives will always handle 2x the IOPS... which most people understand is bad. With 3PAR you even the load up to the point where the smallest drives are full. From that point on, all new writes will only hit the larger drives wit free capacity and as you add more data to the larger drives the IOPS imbalance will increase.

It's simple math. The 3PAR will wide-stripe and the important factor will be IOPS per GB. And with wide-striping the data will be distributed across all drives and you will get in a state where the drives with the lowest IOPS per GB will limit the rest of the tier/cpg. <insert cheesy saying about weakest link/slowest drive>


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